High compression test numbers

[joebillhill]

So i finished up putting a new head on the old girl this week and this morning i did a compression test. I got 500psi all across the board. Is this cause for rejoice or should i be worried about something?
One thing maybe to note is the bottom end was rebuilt 40k km ago and I'm not sure what went into it.

[Growlerbearnz]

The highest "normal" value in the workshop manual is 441psi (for some engines), so something's not right, but it's close enough that I wouldn't panic yet.

First things to check- confirm that your cam timing is correct (all the pointers still line up) and that your valve clearances are all at 0.25mm.

Cylinder head gaskets came in different thicknesses from the factory to correct for manufacturing tolerances in the blocks, but people tend to fit whatever's available when replacing the head. It's possible your block originally had the thickest gasket, and now you've installed a thinner one. (This is in the workshop manual too- the section on replacing the cylinder head talks about counting a row of holes in the gasket, that's how you know what thickness it is).

Worst case: there are two kinds of cylinder heads- protruding-valve and flush-valve, and they have matching pistons. If you've installed a protruding-valve head onto flush-valve pistons the compression would go up, but there's also a risk of the valves touching the pistons at speed. Did you take any photos pf the engine and head while installing it? (See the Technical Reference Library for a thread about heads)

500psi isn't terribly high, it's not going to directly blow up your engine (pressures reach about 1800psi during combustion), but it *is* going to advance the ignition point. Stock timing and boost should be fine, but you want to be cautious with injection timing and inlet temperature, which means not advancing your injection timing too much, and not turning up the boost too much. Either of those things might cause ignition to start while the piston is still coming up the cylinder, which isn't great. At a guess 10-12psi of boost should be ok.

[joebillhill]

Good to know. Thanks GrowlerBear.
I don't have any detailed photos, however my engine is an 89 (or at least that's what out originally was) and the head i had on there before was the early protruding valve style so that's what i searched for when getting the new head. https://www.qcylinderheads.com/products ... ubishi-max
I just got a bare head as the old assembly still looked good.
We adjusted the valves to 0.15 cold but maybe I'll check it warm.
Also I'll take a look at the thinking marks as maybe the belt slipped after startup?
I didn't take a look at the gasket as i thought that was only for the latter style heads. I don't recall seeing those holes but could that be the culprit?
Boost is also just shy of 9psi

[Growlerbearnz]

I didn't take a look at the gasket as I thought that was only for the later style heads. I don't recall seeing those holes but could that be the culprit?

It appears you've read that section of the workshop manual more closely than I have! It does indeed say "engines from July 1993" use variable thickness gaskets, earlier engines it doesn't appear to matter. Huh. Still, the gasket you fitted might have been thinner than the old one. Was it a genuine Mitsubishi gasket, using the part number you looked up to suit your engine?

...and upon further research: it looks like early engines had 4 different piston heights to choose from, while later engines just used one style of piston. I suspect that when an early engine was being built, the factory would compensate for block height variation by installing taller or shorter pistons. Later engines they achieved the same thing by choosing thicker or thinner head gaskets- same result, but far less work.

When your engine was rebuilt, did it have new pistons? Maybe they're taller than the old ones.

We adjusted the valves to 0.15 cold but maybe I'll check it warm.

Rocker clearances change depending on which manual you're reading, I suspect you're following the 1991-'93 workshop manual. Earlier manuals (for earlier engines) will tell you to use 0.25 when hot, but I just set mine to 0.25 when it's cold. When it gets hot the clearances close up by 0.02mm, which isn't worth the hassle of getting hot oil everywhere.

Boost is also just shy of 9psi

You'll have no worries there at all. You can go to 12psi without worry. Stock injection timing should be fine too- a 4D56 responds well to a touch of injection timing advance, and your increased compression will be providing that. It should run really well!

[joebillhill]

Still, the gasket you fitted might have been thinner than the old one. Was it a genuine Mitsubishi gasket, using the part number you looked up to suit your engine?

I think that's the problem. I don't know what my thought process was because it was back in November but i purchased an oem gasket, however it was 1005B426 which looks like it's 150mm whereas the gasket listed for mine is 1005B427 listed at 155mm

I'll loosen those valves up but is there an earlier model manual here? I feel the only ones I've seen are 91-93 or in German

I'll crank that boost up a touch as well

[Growlerbearnz]

I think 1005B426 is 1.45mm (...427=1.50mm ...428=1.55mm). Everything up until 1993 uses a 1005B427 (1.50mm) according to the site at this gigantic URL: https://www.ilcats.ru/mitsubishi/?funct ... anguage=en

Still, yes, it's 0.5mm thinner than it should be. On a nice, fresh, tight engine that might be enough to give you an extra 50psi.

Try the second manual from the downloads section here: http://www.delica.ca/forum/viewtopic.ph ... 35&p=55804
I've updated the descriptions to make it easier to know which manual to follow. I don't have a 4D56 manual for L300s prior to 1991, but this one seems close- the specifications match what's printed on the sticker on the engine cover, at least.

Speaking of which- your engine cover should have a silver sticker on it with valve clearances and injection timing specifications. In Japanese, but the numbers are fairly self explanatory. Like this:

IMG_20180606_120907.jpg (94.67 KiB) Viewed 6062 times

(Timing 7 degrees ATDC (plunger at 1mm lift). Inlet 0.25mm exhaust 0.25mm)

[joebillhill]

Yup i have that identical sticker!
I wonder, when I adjust my valves, will that lower the compression? As there won't be as much air intake?

[Growlerbearnz]

I wonder, when I adjust my valves, will that lower the compression? As there won't be as much air intake?

Hard to predict at cranking (compression-testing) speeds, but any change would be minimal.

Cam/Valve timing is chosen to maximise air intake and exhaust at certain engine speeds* and it does this by working with the velocity of the incoming air stream at that speed, letting the incoming air's inertia ram more air into the cylinder than it normally would (while the exhaust's inertia helps suck exhaust out of the engine). Incorrect valve clearance will alter the valve's timing slightly, which will shift the cam's peak efficiency point. At speeds below that efficiency point the change in timing has less and less effect.

*On our engines it's 2000rpm where the engine's at peak efficiency. Sporty engines aim for higher speeds, and VTEC has both. VTEC is cheating.

[joebillhill]

I've been playing around with the boost. Definitely runs well with the higher boost.

You can go to 12psi without worry.

I'm curious why 12?

[Growlerbearnz]

Because "turning it up to 11" is a cliché?

TL;DR 12psi is about the same efficiency/temperature as 10psi. Above 12psi efficiency rapidly drops off and intake temperature increases, which can be a problem in a high-compression engine.

http://www.delica.ca/forum/viewtopic.php?f=102&t=17650

The turbo's maximum efficiency is 76%. At 10psi, while in that 76% sweet zone, the turbo increases intake temperature by about 100F. At 12psi you can still hit that 76%, and the extra boost only increases intake temperature by another 18F, which is well within the range of natural variation- just driving on a warm day would net the same increase.

At 15psi you start to fall out of the turbo's efficiency peak, and the loss of efficiency plus increase in boost starts to compound the temperature increase. At 15psi, 74% efficiency, the turbo adds 160F to intake temperatures. That's getting significant. And at all other parts of the turbo's range the efficiency is similarly reduced.

Why is this important?

Consider injection and ignition timing in your engine. As the piston rises, compressing the air, the temperature rises. Injection starts before the cylinder is quite hot enough to ignite the fuel; this gives the fuel time to evaporate so it will burn efficiently. Eventually the compressed air is hot enough that the fuel ignites- and if the engine designer has done their job, this happens when the fuel is completely evaporated and ideally distributed through the precombustion and combustion chambers.

If your intake temperature and pressure increase, the fuel will ignite slightly earlier. A more modern engine would detect this and shift the injection timing to compensate. Our engines are more primitive, so they're tuned conservatively with a large safety margin so they can cope with a hot day in the desert.

Once you start changing the boost or compression, you're reducing that safety margin. It would be easy to overdo it: high boost + low efficiency turbo + hot day + climbing a long hill = ignition point advances until the fuel is exploding as it's injected, before it's had a chance to spread out, and before the piston reaches the top of its stroke, which tries to force the piston backwards. Chaos ensues. Expensive chaos.

Which means stick to boost levels which are similar to factory, where you're operating in the turbo's efficiency peak, before temperature increases start compounding.

Or add an intercooler.

[joebillhill]

Or add an intercooler.

Oh it's on the list!